WikiLeaks has begun releasing a cache of what it says are 5.5m emails obtained from the servers of Stratfor, a US-based intelligence gathering firm with about 300,000 subscribers.

The whistleblowing site has published 167 emails in its initial release. WikiLeaks says it has partnered with 25 media organisations around the world, including Rolling Stone, McClatchey, the Hindu and Russia Reporter.

Unlike previous WikiLeaks releases, this latest email cache was apparently obtained through a hacking attack on Stratfor by Anonymous in December 2011 rather than through a whistleblower.

Anonymous published contact and credit card details from Stratfor and said at the time it had also obtained a large volume of emails for which it would arrange publication.

One of the largest Anonymous-linked accounts on Twitter, @AnonymousIRC, put out a series of tweets on Monday morning seemingly confirming it was the source of the WikiLeaks release.

"We promised you those mails and now they'll finally be delivered. Five million (that's 5,000,000) emails at your pleasure," said the Anonymous account."There's a treasure trove of nasty details in those emails. We think there's something for everyone."

WikiLeaks said the documents contained details of the inner workings of the private intelligence agency, links between government and private intelligence, and commentary on WikiLeaks itself.

"The material contains privileged information about the US government's attacks against Julian Assange and WikiLeaks and Stratfor's own attempts to subvert WikiLeaks," the whistleblower website said.

"There are more than 4,000 emails mentioning WikiLeaks or Julian Assange. The emails also expose the revolving door that operates in private intelligence companies in the United States."

The email cache is said to contain information on measures taken to track activist and NGO activity for large companies, through media monitoring, and information on the financial sector.

The hacking attack on Stratfor is subject to an FBI investigation. Several alleged members of Anonymous have been arrested by authorities in the US and UK as part of investigations.

Stratfor had not at the time of writing commented on the authenticity of the published material.

WikiLeaks and some of its media partners – including the Yes Men activists who target Dow Chemicals among others – are scheduled to hold a press conference discussing the release at midday on Monday at the Frontline Club in London.